Scrapie is a slowly progressive, transmissible disease of the CNS in sheep and goats. Scrapie is characterized by a prolonged incubation period averaging 2 years, followed by a clinical course of 2 to 6 months when the animal exhibits sensory and motor malfunction, hyperexcitability, and death. The agent presumably moves from infected to susceptible animals by direct or indirect contact and enters through the gastrointestinal tract. Consequently, its spread appears to be both vertical (mother to offspring in utero) and horizontal (direct contact) between sheep.

The scrapie agent is found in lymphatic tissue (spleen, thymus, tonsil, and lymph nodes) in sheep with preclinical infections; however, in clinically affected sheep, the agent is identified in the intestines, nervous tissues (brain and spinal cord), and lymphatic tissues as determined by experimental infectivity studies in a susceptible animal model. The brain has been demonstrated to have the highest level of infectivity of all tissues.
Scrapie is known to have existed in Britain, Ireland, France, and Germany for over 200 years. It has been observed in the United States and Canada for about 50 years. The first case of scrapie in the United States was diagnosed in Michigan in 1947.